As a means of investigating basic principles that govern the growth of mammalian tissues within the whole animal, we are studying induction of hepatocyte proliferation by surgical, dietary, and hormonal means. The ultimate goal is to identify the combination (s) of extracellular factors that serve as physiologic regulators, and to find how their interaction with liver cells is translated into growth control at the molecular level. Using two animal models--partially hepatectomized rats previously deprived of portal splanchnic viscera and mice lethally injected with murine hepatitis virus--we have found that insulin and glucagon, in combination but not separately, act synergistically with dramatic effectiveness to promote hepatocyte proliferation and protect liver cells against injury. These two hormones, although active potentiators, fail as primary initiators of liver growth. Epidermal growth factor (EGF), however, when infused in combination with either insulin or glucagon, is capable of substantially stimulating hepatocyte proliferation in normal livers of intact rats; stimulation in 10 other organs appear negligible. We propose to continue investigating EGF, insulin and glucagon, and a number of other potentially active hormones and growth factors as well, in various combinations, seeking evidence for key combinations that may be specific physiological regulators of liver growth. We will also explore the conditions or pretreatments that determine the responsiveness of the cells to the various growth regulatory factors, and finally, we will examine relevant biochemical processes, attempting to discover the molecular basis for the effects achieved. These studies will be carried out both in primary monolayer cultures of isolated hepatocytes and in whole animal models in vivo.